1. Field of the Invention
Embodiments of the invention described herein pertain to the field of electric submersible pump (ESP) assemblies. More particularly, but not by way of limitation, one or more embodiments of the invention enable an apparatus, system and method for treatment of an ESP power cable.
2. Description of the Related Art
Submersible pump assemblies are used to artificially lift fluid from underground formations, such as oil, natural gas and/or water wells, to the surface. These wells are typically thousands of feet deep, with the pump assembly placed inside the deep well. A typical electric submersible pump (ESP) assembly consists, from bottom to top, of an electric motor, seal section, pump intake and centrifugal pump, which are all connected together with shafts. The electric motor supplies torque to the shafts, which provides power to the centrifugal pump. The electric motor is generally a two-pole, three-phase, squirrel cage induction design connected by a power cable to a power source located at the surface of the well. The power cable includes a motor lead cable and extension cord, and extends from the downhole motor deep within the well, to the power source at the surface of the well. These ESP power cables are typically between about 4,000 to 12,000 feet in length, depending on well depth, since the cable must extend from the ESP motor deep within the well to the surface where the power source is located.
ESP Power cables conventionally include three insulated copper conductors that are enclosed by a helically wrapped strip of galvanized steel armor. The galvanized steel armor strip on these cables is typically between 20 and 34 mils thick, and the power cable typically weighs about 1.5 pounds per foot. Thus, a 12,000 foot-long power cable weights about 9 tons. When a power cable is new, a zinc coating covers the surface of the galvanized steel armor. The zinc coating protects the cable from rusting before it is deployed. However, during ordinary use of the cable, the zinc coating decays.
ESP power cables are typically the single most expensive component of the ESP assembly. Currently, the cost of an ESP power cable is about $4.00-$12.00 per foot of cable, making the current cost of a 12,000 foot cable as much as $144,000 USD. For this reason, it is often desirable to reuse ESP power cables. In such instances, the cable to be reused is stored between uses. However, since the zinc coating deteriorates during ESP operation, a secondhand power cable quickly rusts when exposed to the elements. Rust decays the galvanized steel armor, causing failure of decompression containment or mechanical protection to the underlying phases, such that the power cable cannot be reused. Conventionally, the shelf life of a gently used power cable is about three to six months.
One approach to extending the shelf life of power cables is to wrap the power cable in a sheet during storage in order to protect the cable from the elements. However, rudimentary wrapping has failed to significantly reduce degradation due to rust. Another approach has been to pull the cable through a rust inhibitor by unspooling the cable, pulling it through the rust inhibitor, and then respooling the cable onto a new reel. But unspooling the cable, pulling it, and respooling has proven difficult to implement and labor intensive. Since the cable is up to 12,000 feet long and nine tons heavy, the cable is difficult to handle, particularly once it is unwound off the reel. In addition, this unspooling process takes up a large amount of space.
Yet another approach has been to use a crane to submerge the cable in a pit full of rust inhibitor. This undesirably requires a large pit and a large quantity of rust inhibitor to cover 12,000 feet of cable—about 2,500 gallons of rust inhibitor—and much of the rust inhibitor is spilled or wasted in the process. Furthermore, overhead cranes are expensive and often not readily available, and submerging a spooled cable often fails to coat the entire cable, since air bubbles become trapped in the cable string and prevent the rust inhibitor from being applied to those areas.
As is apparent from the above, current ESP power cables are not adequately protected from degradation due to rust, and current attempts to apply rust inhibitors to ESP cables are expensive, wasteful and difficult to implement. Therefore, there is a need for an apparatus, system and method for treatment of ESP power cables to improve the shelf life of the cables and the feasibility of rust treatment techniques.